Flow control of exhaust gas through an internal combustion engine has been used in order to provide vehicle engine braking of both the compression-release type and the bleeder type. Both types of engine braking operate by decompressing an engine cylinder to allow exhaust gas to exit the cylinder. Control of the flow of exhaust gas may also provide benefits during engine start-up. Specifically, holding open an exhaust valve during engine start-up may decompress the cylinder so that the piston may move towards a cylinder top dead center (TDC) position more easily. Benefits from decompression during engine start-up may include easier engine starting, lighter starting system and/or battery requirements, and avoidance or reduction in the need for additional starting aids.
Generally, engine braking systems may control the flow of exhaust gas from the engine cylinders to the exhaust system (i.e., exhaust manifold, tail pipe, etc.). The flow of exhaust gas from the engine cylinders may be controlled to provide a retarding force on the engine pistons to slow the engine. Specifically, one or more exhaust valves may be selectively actuated to provide compression-release, bleeder, and/or partial bleeder engine braking.
The operation of a compression-release type engine brake, or retarder, is well known. A four-stroke internal combustion engine experiences intake, compression, expansion, and exhaust cycles during its operation. The intake cycle occurs in conjunction with a main intake valve event, during which the intake valves in each cylinder are opened to allow air to enter the cylinder. The exhaust cycle occurs in conjunction with a main exhaust valve event, during which the exhaust valves in each cylinder are opened to allow combustion gases to exit the cylinder. Typically, the exhaust and intake valves are closed during much of the compression and expansion cycles. During compression-release engine braking, fuel supply to the engine cylinders is ceased and, in addition to the main exhaust valve event, one or more exhaust valves also may be selectively opened during the compression stroke to convert the internal combustion engine into a power absorbing air compressor. Specifically, as an engine piston travels upward during the compression stroke, the gases trapped in the cylinder are compressed and oppose the upward motion of the piston. As the piston approaches the top dead center (TDC) position during the compression stroke at least one exhaust valve may be opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the piston on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference.
The operation of a bleeder type engine brake is also known. During bleeder engine braking, in addition to the main exhaust valve event, one or more exhaust valve(s) may be held slightly open throughout the remaining engine cycles (i.e., the intake, compression, and expansion cycles for a full-cycle bleeder brake) or during a portion of the remaining engine cycles (i.e., the compression and expansion cycles for a partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the former may permit the exhaust valve to close during most or all of the intake cycle. An example of a bleeder engine brake is disclosed in Yang, U.S. Pat. No. 6,594,996 (Jul. 22, 2003), which is hereby incorporated by reference.
The initial opening of the exhaust valves in a bleeder braking operation may be in advance of TDC of the compression stroke, and is preferably near a bottom dead center (BDC) point between the intake and compression cycles. As such, a bleeder type engine brake may require much lower force to actuate the valves, and generate less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake. Thus, an engine bleeder brake can have significant advantages.
An engine decompression system may hold open one or more exhaust valves in an engine cylinder during the start-up of the engine. An engine decompression system of the type described herein may be particularly useful in cold weather conditions, when cranking battery power is lower, cranking time to start-up is increased, and the engine is more difficult to turn over. In addition, engine decompression, which may reduce battery power and starter system requirements, may result in lower weight components, which permit increased fuel efficiency. Reduction in start up time resulting from use of a decompression system may also provide emissions benefits. Accordingly, advantages such as these, but not limited to the foregoing, may be realized by use of one or more of the embodiments of the invention described herein.
Additional advantages of various embodiments of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.